1. Field of the Invention
This invention relates to a method of preventing agglomeration of carbonaceous solid particles in a fluid bed in coal conversion processes. More particularly, this invention relates to improvements in a process for reacting coal particles separately introduced liquid hydrocarbons in a fluid-bed hydrocarbonization zone.
2. Description of the Prior Art
Increasing energy needs have focused attention on solid fossil fuels due to their availability in the United States in a relatively abundant supply and their potential value if converted into more useful forms of energy and feedstock. Processes such as carbonization, gasification, hydrocarbonization and hydrogasification, wherein synthetic fuel products have been prepared by introducing a fluidized stream of finely-divided coal particles into a fluid-bed reaction zone and reacting the coal particles at elevated temperatures in the presence of inert gases, air, steam, hydrogen or the like, are well known. A major operating difficulty in such processes has been the tendency of coal particles, especially intensified in a hydrogen-rich atmosphere, to agglomerate at the elevated temperatures required for reaction.
Coal particles, especially caking, swelling or agglomerating coals, become sticky when heated in a hydrogen-rich atmosphere. Even non-caking, non-swelling and non-agglomerating coals become sticky when heated in such an atmosphere. Coal particles begin to become sticky at temperatures in the range of about 350.degree. C. to about 500.degree. C., depending on the specific properties of the coal, the atmosphere and the rate of heating. The stickiness results due to a tarry or plastic-like material forming at or near the surface of each coal particle, by a partial melting or decomposition process. On further heating over a time period, the tarry or plastic-like material is further transformed into a substantially porous, solid material referred to as a "char." The length of this time period, generally in the order of minutes, depends upon the actual temperature of heating and is shorter with an increase in temperature. By "plastic transformation" as used throughout the specification is meant the hereinabove described process wherein surfaces of coal particles being heated, particularly when heated in a hydrogen atmosphere, develop stickiness and transform into substantially solid char, non-sticky surfaces. "Plastic transformation" is undergone by both normally agglomerating coals and coals which may develop a sticky surface only in a hydrogen-rich atmosphere.
Agglomerating or caking coals partially soften and become sticky when heated to temperatures between about 350.degree. C. to about 500.degree. C. over a period of minutes. Components of the coal particles soften and gas evolves because of decomposition. Stick coal particles undergoing plastic transformation tend to adhere to most surfaces which they contact such as walls or baffles in the reactor, particularly relatively cool walls or baffles. Moreover, contact with other sticky particles while undergoing plastic transformation results in gross particle growth through adherence of sticky particles to one another. The formation and growth of these agglomerates interferes drastically with the maintenance of a fluid-bed and any substantial growth usually makes it impossible to maintain fluidization.
In particular, entrance ports and gas distribution plates of equipment used in fluid-bed coal conversion processes become plugged or partially plugged. Furthermore, even if plugging is not extensive, the sticky particles tend to adhere to the walls of the vessel in which the operation is conducted. Continued gross particle growth and the formation of multi-particle conglomerates and bridges interferes with smooth operation and frequently results in complete stoppage of operation.
Agglomeration of coal particles upon heating depends on operating conditions such as the heating rate, final temperature attained, ambient gas composition, coal type, particle size and total pressure. When heated in a hydrogen atmosphere, even non-agglomerating coals, such as lignites or coals from certain sub-bituminous seams, are susceptible to agglomeration and tend to become sticky in a hydrogen atmosphere. Thus, agglomeration of coal particles is accentuated in a hydrocarbonization reactor where heating in the presence of a hydrogen-rich gas actually promotes formation of a sticky surface on the coal particles reacted. Moreover, in general, introducing any carbonaceous, combustible, solid particles, even those that are normally non-agglomerating, to a fluid-bed having an atmosphere tending to induce agglomeration can result in agglomeration and defluidization of the bed.
Heavy liquid materials are also fed at times to the fluid-bed in coal conversion processes. They may be recycled heavy tar products to be converted to lower molecular weight products, light liquids and gases. Or they may be heavy liquids from an external source added, for example, to enrich the normal gas and/or liquid product, or as a means of waste disposal. Feeding such liquids is known to cause rapid loss of fluidization due to particle agglomeration and plugging.
In an attempt to overcome the problems associated with agglomeration, char as a recycle material from fluidized bed processes has been mixed with an agglomerating type coal feed at a ratio as high as 8 to 1. Also, tar has been ball-milled with a great excess of adsorbent char before feeding into the processes. However, since such procedures reduced the unit throughput, they are wasteful of energy and therefore costly. Other attempts included a pretreatment step wherein coal was oxidized and/or devolatilized superficially in order to prevent sticking and agglomeration of particles, but this lowered yields of useful products and added costs. Thus, it is highly desirable economically to avoid or at least reduce the extent of such pretreatment or such char recycle.